Joining the ends of a tension spring to make a 'drive band'

[Grommet]

The spring is 1m long, close coiled, Ø6.0mm OD Ø1.0mm wire and made from C51000 Phosphor Bronze, 5% Grade "A" Spring Temper H08.
It is joined with a 'spring' made from the same material that is 30mm long and Ø4.27mm OD (to be a tight fit when 'screwed' into the Ø6.0mm spring).
The assembled band runs on two Grade 304 stainless pulleys Ø272mm and Ø154mm at a center distance of 250mm.
The pulleys are driven seperately by a poly-vee drive belt so the spring carries no load from the drive system.
The smaller pulley rotates at 1400rpm.
The spring is used as an electrical conductor.
Now the problem: Two modes of failure are occurring,
1) The smaller joining spring snaps (fatigue).
2) The OD of the smaller spring and ID of the larger spring wear allowing the joint to fall apart.
Question. What joint methods could be used that will not fail?

 

[unclesyd]

All spring drives that I've seen have modified hook and eye. I can't recall seeing a spring drive operating at this speed.

Nearly every 16 mm movie projector used spring drives and all were of the hook and eye variety.

I've got 2 older model fathometers and both have a spring drive on the paper drives and are of the hook and eye connection.

 

[israelkk]

The spring does suffer loads because it changes position from a bend to radius of 77mm to a straight position when it moves in and out of the pulley. Did you calculate the stresses in both springs?

A similar connection is used in Garter springs but they are usually used for static applications.

 

[Grommet]

Thanks for the two responses. To add detail to my original post, I should add that to prevent the 'joiner' spring migrating into either end of the main spring during operation, a single spring coil from a donor main spring was cut and soldered to the middle of the joiner. When joining the spring, the joiner was wound fully into one end of the main spring, then the main spring was 'unwound' a number of turns and then allowed to wind back onto the joiner.
I have found the soldered ring and the close-joined main spring seems to be the problem, as the joint is too stiff. I am now testing a joint section where the ends of the main spring are further apart and the joiner does not have a soldered coil.

 

[mloew]

Is this a potential application for a "machined spring"?

http://www.heli-cal.com/Html/Products/Springs.htm

Best regards,

Matthew Ian Loew


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